i would say the mechanic is correct,probably got some moisture in the points and plugs causing it to stall,
as to the lockup i think this would be a compression lockup if it was a dry road it would have just chugged to a stop.
but seeing as it was wet your wheels would have broke traction and started to aquaplane causing the skid.
nothing you could have done to stop it..
on a whole i would say there was nothing actually wrong with your car.cars frequently stall in wet weather.